A magnetic disk for a hard disk drive is known as one type of information recording medium. The magnetic disk is a disk having a center hole and is fabricated by superimposing magnetic films on the surface of a glass substrate. The magnetic disk is rotated by a spindle received in the center hole. Information recorded on the magnetic disk is read by a magnetic head, which moves along the surface of the magnetic disk in a state levitated from the surface by a certain distance.
It is desirable for the magnetic disk to have a high recording density to increase the recording capacity of the magnetic disk. Japanese Laid-Open Patent Publication No. 2002-150547 describes a method for smoothing the surface of a glass substrate to decrease the distance between the magnetic disk surface and the head and increase the recording density of the magnetic disk. More specifically, an abrasive that chemically affects the glass substrate, such as cerium oxide, is used to polish and smooth the surface of the glass substrate. An acid solution is used to remove foreign articles such as iron particles and abrasive particles adhered to the smooth surface (acid washing). Then, the surface of the glass substrate is etched with an alkaline solution (alkaline washing). The etching removes about 10 nm of the glass substrate.
Due to the demand for a magnetic disk having a higher recording density, there is a tendency for further decreasing the distance between the surface of the magnetic disk and the magnetic head. However, with the magnetic disk manufactured through the conventional method, the magnetic head cannot move further closer to the magnetic disk. More specifically, when alkaline washing removes about 10 nm of the glass substrate, the manufactured glass substrate may have deficiencies such as the etching being uneven, protuberances of abnormal heights being produced on the glass substrate, and the surface state differing locally (differences in the shapes of valleys and peaks). A magnetic disk manufactured from a deficient glass substrate has a tendency of causing a deficiency (glide error) such as the moving head crashing against or being caught by an abnormal protuberance.
To solve this problem, a weak acid solution or weak alkaline solution may be used when performing the washing. However, many iron particles and abrasive particles are chemically and firmly adhered to or caught in the surface of the glass substrate. Thus, all of the iron particles and abrasive particles cannot be washed off when using a weak acid solution or weak alkaline solution. In some cases, this would lower the cleanness of the glass substrate.
As another way to remove the iron particles and abrasive particles, the surface of the glass substrate may be polished with an abrasive after the acid washing. However, this method is not preferable in that the abrasion grains contained in the abrasive may adhere to the glass substrate and in that the surface of the glass substrate may be deformed by performing washing after the polishing.